Tractors are known which comprise a transmission system including a continuously variable transmission (CVT) for transmitting a torque from an internal combustion engine to a driving axle and hence to the wheels. Known continuously variable transmissions may comprise a hydrostatic transmission including a hydrostatic unit. The latter in turn may comprise a variable displacement hydraulic pump connected to a hydraulic motor. By varying the displacement volume of the hydraulic pump, the wheel speed can be set to the desired value.
In addition to the hydrostatic transmission, the transmission system of known tractors also comprises a planetary gearing interposed between the engine and the wheels.
Different torque paths may be defined for transmitting torque from the engine to a location downstream of the transmission system, depending on the gear ratio selected by the driver. These torque paths may either pass through the hydrostatic transmission or not, or they may involve different gears in the planetary gearing.
A power take-off may be provided downstream of the transmission system. One or more implements can be connected, if desired, to the power take-off and receive power from the engine through the transmission system.
Known tractors may comprise a power boost device for requesting additional power from the engine when an implement is connected to the power take-off, in order to ensure that enough power is nevertheless available to the wheels. The power boost device is activated when torque at the power take-off exceeds an on-threshold value and is deactivated when torque at the power take-off drops below an off-threshold value.
In order to activate and deactivate the power boost device at the proper moment, it is therefore important to estimate the current value of the torque at the power take-off.
To this end, calculation methods have been provided which try to estimate the torque at the power take off on the basis of two parameters, i.e. a torque value measured upstream of the transmission system and a twist value measured between two points of a driveshaft of the transmission system. The twist value is an angle which is indicative of the phase difference between the rotational speed downstream and upstream of the transmission system.
The known calculation methods have the drawback that they are quite inaccurate, particularly at certain working points.